Experimental Consideration, Treatments, and Methods in Determining

Published April 8, 2014

Review & Analysis–Soil & Water Management & Conservation

Experimental Consideration, Treatments, and Methods in Determining Soil Organic Carbon Sequestration Rates Kenneth R. Olson*

Dep. of Natural Resources and Environmental Sciences College of Agricultural, Consumer and Environmental Sciences Univ. of Illinois 1102 Goodwin Ave Urbana, IL 61802

Mahdi M. Al-Kaisi

Dep. of Agronomy College of Agriculture and Life Sciences Iowa State Univ. 2104 Agronomy Hall Ames, IA 50011

Rattan Lal

School of Environment and Natural Resources The Ohio State Univ. Columbus, OH 43210

Birl Lowery

Dep. of Soil Science College of Agric. and Life Sciences Univ. of Wisconsin-Madison 1525 Observatory Dr. Madison, WI 53706

In agricultural land areas, no-tillage (NT) farming systems have been practiced to replace intensive tillage practices such as, moldboard plow (MP), chisel plow (CP), and other systems to improve many soil health indicators, and specifically to increase soil organic carbon (SOC) sequestration and reduce soil erosion. Numerous approaches to estimate the amounts and rates of SOC sequestration as a result of a switch to NT systems have been published, but there is a concern regarding protocol for assessing SOC especially for different tillage systems. Therefore, the objectives of this paper are to: (i) define and understand concepts of SOC sequestration, (ii) quantify SOC distribution and the methodology of measurements, (iii) address soil spatial variability at field- or landscape-scale for potential SOC sequestration, and (iv) consider proper field experimental design, including pretreatments baseline for SOC sequestration determination. For SOC sequestration to occur, as a result of a treatment applied to a land unit, all of the SOC sequestered must originate from the atmospheric CO2 pool and be transferred into the soil humus through land unit plants, plant residues, and other organic solids. The SOC stock present in soil humus at end of a study must be greater than the pretreatment SOC stock levels in the same land unit. However, one should recognize that a continuity equation showing drawdown in atmospheric concentration of CO2 may be difficult, if not impossible, to quantify. Therefore, SOC sequestration results of paired comparisons of NT to other conventional tillage systems with no pretreatments SOC baseline, and if the conventional system is not at a steady state, will likely be inaccurate where the potential for SOC loss exists in both systems. To unequivocally demonstrate that the SOC sequestration has occurred at a specific site, a temporal increase must be documented relative to pretreatment SOC content and linked attendant changes in soil properties and ecosystem services and functions with proper consideration given to soil spatial variability. Also, a standardized methodology that includes proper experimental design, pretreatment baseline, root zone soil depth consideration, and consistent method of SOC analysis must be used when determining SOC sequestration. Abbreviations: BMPs, best management practices; CP, chisel plow; GHG, greenhouse gas; IPCC, Intergovernmental Panel on Climate Change; MP, moldboard plow; MRT, mean residence time; NT, no tillage; PT, plow tillage; SIC, soil inorganic C; SOC, soil organic carbon; SOM, soil organic matter; TSC, total soil carbon; WL, woodlot.

Definition of Soil Organic Carbon Sequestration Terrestrial carbon (C) sequestration can be defined as the capture and secure storage of atmospheric C into biotic and pedologic C pools that would otherwise be emitted to or remain in the atmosphere (Lal, 2007). The idea of C storage is: (i) to prevent C emission caused by human activities from reaching the atmosphere by capturing and diverting it to secure storage, or (ii) to remove it from the atmosphere by various means and to enhance its mean residence time (MRT) in the Soil Sci. Soc. Am. J. 78:348–360 doi:10.2136/sssaj2013.09.0412 Open Access Received 24 Sept. 2013. *Corresponding author ([email protected]). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Soil Science Society of America Journal

soil. Soil organic carbon (SOC) sequestration is also defined by Olson (2013a; p. 203) as process of transferring CO2 from the atmosphere into the soil of a land unit through unit plants, plant residues and other organic solids, which are stored or retained in the unit as part of the soil organic matter (humus). Retention time of sequestered carbon in the soil (terrestrial pool) can range from short-term (not immediately released back to atmosphere) to long-term (millennia) storage. The sequestrated SOC process should increase the net SOC storage during and at the end of a study to above the previous pre-treatment baseline.

questration, and (iv) consider field experimental design including pretreatments baseline for determination of SOC sequestration.

Literature Review

Because of burning fossil fuels, cultivation and draining of grasslands and wetlands, deforestation, and land-use conversions, there has been increased interest in developing methods to sequester atmospheric CO2 (Sundermeier et al., 2005; Baker et al., 2007; Lal, 2009). In agricultural land areas, NT systems have been proposed as an alternative to replace intensive tillage systems such as MP and CP as a way to sequester SOC (Ogle et al., 2012; Luo et al., 2001). Many researchers ( Johnson et al., 2005; Liebig et al., 2005; Franzluebbers, 2005; Franzluebbers Carbon sequestration by agricultural land has generated and Follett, 2005; Lal et al.,1998; Kumar et al., 2012) have suginternational interest because of its potential impact on and bengested that converting from MP or CP systems to NT has a efits for both agriculture and climate change adaptation and mitlarge potential for SOC sequestration, but NT does affect the igation. Where proper soil and residue management techniques distribution of SOC within the soil profile (Baker et al., 2007; are implemented, agriculture can be one of many potential Luo et al., 2010; VandenBygaart et al., 2003; Sundermeier et al., amelioration practices to alleviating the problem of greenhouse 2005; Ogle et al., 2012). When NT, CP, and MP are under a gas (GHG) emissions. Additionally, agriculture conservation non-steady state (Olson, 2010) all three tillage systems may fail practices (e.g., the use of different cropping systems and plant to sequester a significant amount of SOC. Conservation tillage and NT systems along with crop roresidue management, as well as organic management farming) tation have been implemented to maximize soil C storage or can enhance soil C storage. Farmers, as well as the soil and environment, receive benefits from C sequestration. Agricultural SOC sequestration. Lal et al. (1998) suggested that conversion ecosystems represent an estimated 11% of the earth’s land surface of a conventional system to NT could result in a 0.50 Mg or 106 g C ha-1 yr-1 sequestration rate, and West and Post (2002) (USDA–NRCS, 1996, 1998) and include some of the most profound from global analysis of long-term agricultural manageductive and C-rich soils. As a result, they play a significant role ment experiments that conversion of plow tillage (PT) to NT in the storage and release of C within the terrestrial C cycle (Lal can sequester 0.57 Mg C ha-1 yr-1 over 15 to 22 yr. Results from et al., 1995). The negative and positive considerations of the soil another study suggested that a NT system sequestered SOC in C balance and emission of GHGs from the soil are: (i) potential increase of CO2 emissions from soil contributing to the increase the top 0- to 20-cm layer at 0.19 Mg C ha-1 yr-1 for Hoytville soil of radiative force, (ii) the potential increase in emissions of other and 0.17 Mg C ha-1 yr-1 for Wooster soil in Ohio, USA (Kumar GHGs (e.g., N2O and CH4) from soil as a consequence of land et al., 2012). management practices and fertilizer use, and (iii) the potential While Johnson et al. (2005), Liebig et al. (2005), and for increasing C (as CO2) storage into soils, which equals 1.3 Franzluebbers (2005) reviewed and synthesized the results to 2.4 Pg or 1015 g C yr-1 (Tans et al., 1990), and to help reduce for the central, northwestern, and southeastern regions of the future increases of CO2 in the atmosphere. USA, there are questions as to the steady state of the soils reThe objectives of this review paper are to: (i) define and unported (Table 1). These specific regional SOC sequestration derstand concepts of SOC sequestration, (ii) quantify SOC disrates were apparently determined based on many comparison tribution and the methodology of measurements, (iii) address soil studies within each region (Christopher et al., 2009) with the spatial variability at field- or landscape-scale for potential SOC seSOC concentration measured at the end of the tillage study and it was reported that SOC stock was greater in soil under NT than those under MP. These findings Table 1. Published Worldwide and Regional USA soil organic C (SOC) retention rates or of SOC sequestration were based on rates of net SOC storage change for a switch from conventional till (CT) to no-till (NT). results of paired comparisons of NT to Region, USA Worldwide SOC Retention Rate (Mg C ha-1 yr-1) Source reference other conventional tillage systems with North Central 0.48 ± 0.59 Johnson et al. (2005) no pretreatments baseline SOC conNorthwest 0.27 ± 0.19 Liebig et al. (2005) centration measurement and assuming South East 0.45 ± 0.04 Franzluebbers (2010) that conventional system baseline was North East -0.07 ± 0.27 Gregorich et al. (2005) at steady state. Changes in SOC conSouth West 0.30 ± 0.21 Franzluebbers and Follett (2005) centration with changes in manageUSA 0.50 Lal et al. (1998) ment can either be related to enhanced Worldwide 0.48 ± 0.13 West and Post (2002) tillage disturbance or changes in C Humid climates 0.22 Six et al. (2004) inputs. Carbon inputs from outside a Arid climates 0.10 Six et al. (2004) www.soils.org/publications/sssaj

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land unit, such as animal waste or other C sources, are considered to be a redistribution of C already in storage and from external sources and not sequestered SOC. These C rich amendments need to be accounted for when they are applied to the land unit. The impact of tillage disturbance applied to a land unit on SOC concentration has been documented. Land unit crop yield, biomass, and residue returned to the soil are usually not significantly different as a result of the tillage treatment (Olson et al., 2013). If the residue returned to the land unit soil is significantly different between tillage treatments and resulted in humus formation increase it could be as a result of SOC sequestration. When the impacts of tillage treatments on SOC sequestration are measured, the C inputs are often statistically similar for all tillage treatments. Therefore, only the effects of tillage disturbance which does have a significant effect on SOC loss were considered. Balesdent et al. (1990) compared PT with NT and observed that PT practice enhances SOC and N mineralization by incorporating crop residues, disrupting soil aggregates and increasing aeration causing a reduction in SOC and N stocks. Plowing was the primary cause of SOC oxidation and emission of CO2 to the atmosphere (Al-Kaisi and Yin, 2005a). Drainage of poorly drained, nearly level soils (Olson et al., 2002) and well drained, sloping and eroded soils (Olson, 2010) often resulted in the accelerated rates of SOC oxidation and emission of CO2. Kern (1994) estimated historical soil C losses in the top 30 cm after cultivation for major field crops in the contiguous USA to be 16% primarily as a result of plowing which caused oxidation of SOC and emissions of CO2 to the atmosphere. Franzluebbers and Follett (2005) reported that the SOC stock of timberland and prairie soils declined with cultivation in North America. The SOC rate of decline as a result of cultivation was 22 ± 10% for the northeast, 34 ± 24% for the northwest, no value was reported for the central, 25 ± 33% for southwest, and 36 ± 29% for southeast. Many researchers (Clark et al., 1985; Lal, 2003) have reported that accelerated soil erosion exacerbates C emissions. Lal (1995) suggested that 20% of the C translocated by erosion may be released eventually as CO2 into the atmosphere, 10% to transport as dissolved C in water bodies, and 70% remains on the landscape often on a lower landscape position. Van Oost et al. (2007a) found that the eroded C gets buried at the bottom of the slope where decomposition rates are slower. According to Baker et al. (2007), this difference in SOC distribution can be attributed to different properties of PT vs. NT soils. In general, NT soils are more compacted with greater bulk density, particularly at the soil surface, than recently plowed soils (Kitur et al., 1993). Furthermore, crop residues left on the soil in NT causes a decrease in soil temperature early in the growing season, which limits root growth (Baker et al., 2007). West and Post (2002) reviewed 137 paired studies, but in the studies considered, SOC concentration was only measured in the top 15 or 30 cm of soil (Baker et al., 2007). Another review by VandenBygaart et al. (2003), however, included studies where SOC concentration was measured to a depth > 30 cm. 350

The only difference found was the location of the C accumulation within the soil profile. In NT plots, SOC stocks were concentrated in the top 30 cm, but were dispersed to greater depths in tilled plots (Baker et al., 2007; Blanco-Canqui and Lal, 2008; VandenBygaart et al., 2003). The many land-use and agricultural system changes over time have had an impact on the SOC concentrations and trends (gains, steady-state, or loss) in the USA. Each change affected the SOC concentrations and trends. Some management practices increased SOC stocks and some reduced them, but SOC concentration seldom reached a true “steady state” since many of these land use and practice changes were adopted at various times and had an impact over different periods (Fig. 1). It usually takes a significant number of years to reach a steady state, especially if the soils are sloping and eroding and SOC-rich sediment is being transported from the land unit (Guzman and Al-Kaisi, 2010). The real issue that needs to be clarified here is the concept of SOC sequestration as related to the system characteristics, such as NT (Schlesinger, 1999). There is some validity about NT superiority to intensive tillage in terms of SOC retention. However, annual losses of C from SOC storage as a result of oxidation and emissions of CO2 to the atmosphere at specific sites could be greater than the additional amount of annually stored SOC (in humus) by any agricultural system (Olson, 2010). Thus, the SOC sequestration concept needs to be applied to a specific land unit by establishing boundaries such as pretreatment baseline to ensure rigorous measurements and accurate interpretation of findings in evaluating the effectiveness of management practices in removing CO2 from the atmosphere and positively increasing the SOC stock above the baseline (Fig. 2) or with reference to the SOC stock before the establishment of treatments. While the NT 0- to 5-cm layer sequestered SOC, the 0- to 75cm root zone layer lost SOC (above a root restricting fragipan with only trace amounts of SOC) during the 20 yr. In addition to the management and land-use effects on SOC retention, the soil sampling protocol must account for spatial variability.

Fig. 1. Soil organic carbon content over a 2- to 14-yr period of reconstructed prairies and potential future increase (Guzman and AlKaisi, 2010: J. Environ. Quality 39:136-146). Soil Science Society of America Journal

Soil Carbon Distribution with Depth When determining SOC sequestration, storage, or retention it is important to include the entire root zone, which is commonly to a depth of between 1 and 2 m unless there is a root restrictive layer present, such as a very dense horizon, fragipan, or bedrock. Tillage systems can influence SOC distribution and storage or retention in the surface and subsurface layers. Deep tillage, such as MP or chisel, can significantly alter SOC distribution in the soil profile. Soil inversion can completely translocate surface soil SOC to lower depths (Fig. 3). In a long-term tillage study in Illinois (Fig. 2), the NT system showed SOC increase or sequestration in the upper 0- to 5-cm layer, but there was a SOC loss within the 5- to 75-cm layer. The SOC stocks need to be accounted for in the entire root zone to assess tillage system effect and plant contributions to SOC change. Because of the expense and effort involved, most soil sampling techniques focus on SOC measurements within the soil surface layer as a primary metric for evaluating SOC change. The soil surface however is a reliable marker only for measurement of C concentration characteristics as directly related to a C concentration characteristic below the soil surface at the time of sampling (Wuest, 2009). Deeper than surface soil sampling will not completely overcome a bias caused by bulk density variations and various other soil variables including change in soil surface elevation except when SOC concentration is universally absent (approaching zero) at lower depths. Sampling soils

Fig. 2. Soil organic carbon levels in 0- to 5-cm and 0- to 75-cm layers in Grantsburg soils during a 20-yr tillage experiment. www.soils.org/publications/sssaj

to a depth that the SOC concentration is universally absent is recommended (Lee et al., 2009; Wuest, 2009). In most soils, SOC concentration is approaching zero but trace amounts are still present which does not significantly change the root zone total SOC stock in the soil profile at a depth ranging from 0.75 to 1.0 m (Soil Survey Staff, 1968). Equivalent soil mass (massdepth) instead of linear depth can be used to correct for tillage treatment differences in soil bulk density allowing more precise and accurate quantitative comparison of SOC stocks (Lee et al., 2009; Wuest, 2009; Ellert and Beltany, 1995). Doetterl et al. (2012) proposed an alternative method to measure soil variability with depth. The SOC data should be expressed on equal soil mass per unit area to appropriate depth where SOC concentration is absent. An example of such units would be Mg C ha-1 to a 1-m depth.

Methods to Separate Soil Organic Carbon from Soil Inorganic Carbon and Total Soil Carbon

Methods used to quantify SOC concentration can be used to distinguish soil inorganic C (SIC) from SOC. The latter can be measured by dry combustion methods (Soil Survey Staff, 2004) if HCl pretreatment is used to eliminate the SIC before ignition and measurement where the total soil carbon (TSC) value becomes the SOC value (Harris et al., 2001). Soil pH greater than 7.1 has been used by researchers (David et al., 2011) to identify samples with SIC and selected for pretreatment with HCl. The SOC can be measured by wet chemistry method as well as modified acid-dichromate organic C procedure (Soil Survey Staff, 2004). The accuracy of this method is less than CN analyzers since oxidation of organic C is incomplete and requires the use of a correction factor which varies with conditions (Lettens et al., 2007; Meersmans et al., 2009). Another method for determining SIC if the soil pH was > 7.1 is by using a modified pressure calcimeter method (Sherrod et al., 2002; Al-Kaisi

Fig. 3. Soil organic carbon changes with depth in Grantsburg soils after 20-yr of tillage treatments. The baseline for the no tillage and moldboard plots were statistically similar and mean was used as baseline for both treatments. 351

and Grote, 2007) and subtract from the TSC concentration values determined initially by the dry combustion. An alternative approach, when carbonates are high would be to measure SIC or carbonate and subtract this from TSC using dry combustion to determine the SOC by difference (Soil Survey Staff, 2004). When SOC is measured multiple times in the course of a long-term study, the same laboratory method should be used (Mulvaney et al., 2010). Researchers should not assume soil samples do not have SIC sources, and should take under consideration past liming or naturally formed carbonates even if the soils samples have a pH below 7.0 and then report their total C findings (dry combustion as SOC when no pretreatment with HCl was used [Mulvaney et al., 2010]). This is especially critical consideration if the intention is to determine soil C sequestration and to be in compliance with the definition, any additional carbons (i.e., organic manure, inorganic natural occurrence such CaCO3, or as an amendment as liming) must be assessed to determine SOC change during the study.

Soil Spatial Variability Consideration

Natural soil spatial variability occurs at both large and small scales including on almost all plot areas regardless of size and it should be addressed in the experimental design with sufficient replication (Olson and Kitur, 1993). Natural variability in soils will be reflected in different soil parameters and the outcome can vary significantly, if sampling is insufficient to account for such variability (Olson et al., 1985). Different soil texture and type of clay mineralogy can have significant effects on SOC retention. Increase in clay content has been shown to increase SOC concentration (Bationo et al., 2007) and type of clay whether it is 1:1 or 2:1 can have an effect on SOC retention or storage. Careful evaluation of soil before establishing studies dealing with evaluating SOC sequestration with different tillage and cropping systems is essential. Sequestration of SOC as it was previously defined depends on the interaction between soil and plant. Therefore, soil differences can be reflected in plant performance, productivity, and biomass production as main contributors to SOC input in the soil system. It is helpful to document and demonstrate spatial variability utilizing tools such as threedimensional (3-D) map of the soil to understand the interaction effects of land management practices and soil type on SOC distribution (Grunwald et al. 2001a, 2001b, 2001c; Arriaga and Lowery, 2005). The use of a soil sampling grid that accounts for surface variability, but also accounts for below soil surface variability within the root zone will provide more accurate accounting for SOC stocks changes or retention.

Microbial Role in Soil Organic Carbon Sequestration or Net Storage

To understand SOC sequestration it is imperative to shed light on the role of soil microbial activities and how they influenced by management practices such as cropping and tillage systems and subsequent effects on SOC sequestration (Lupwayi 352

et al., 1999; West and Post, 2002). Soil microbial activity and biodiversity are essential components for any agro-ecosystem to sufficiently sequester SOC. Soil microbial contribution to soil organic matter (SOM) is influenced by the microbial community size, dynamics, and process of formation and decomposition that influence their stability (Six et al., 2006). However, changes in tillage and cropping systems are some of agricultural practices can influence the dynamic microbial activities, which ultimately affect SOC dynamic and stability (Scow, 1997; Paustian et al., 1997). Soil organic C change is essentially determined as a balance between organic C input from above and below ground plant sources and organic C losses as result of decomposition, erosion, and leaching, which influenced by soil topography, drainage class, and management systems (i.e., tillage and crop rotations). As soil microbial activity influenced by tillage and cropping systems, SOC change will eventually be affected as demonstrated by root and basal respiration rate, where significant differences have been observed between different tillage and cropping systems (Wardle, 1995; Guzman and Al-Kaisi, 2010). During the respiration and decomposition process, which will be accelerated by intensity of tillage system, the C in plant residue and some in various other soils’ C pools are released to atmosphere as CO2. This release rate of CO2 can be accelerated over time as more intensive tillage takes place, which can influence SOC stability (i.e., steady state of SOC; Cole et al., 1997). Intensive tillage increases soil aeration resulting in increased residue decomposition due to increase in biological activities. Since soil microbial communities play a significant role in regulating SOC dynamics, any shift in such activities in response to different tillage and cropping systems (Fig. 1) play critical role in determining the outcome and rate of SOC gain or loss as CO2. Determining the quantity of atmospheric CO2 sequestered in the soil depends on the stability of the agriculture system (no disturbance), metabolism of the ecosystem, where atmospheric CO2 is fixed by plants, and converted into organic C compounds through photosynthesis as a mechanism for C sequestration. During SOC decomposition, CO2 is released through heterotrophic respiration, which is strongly correlated with soil temperature and moisture regimes (Linn and Doran, 1984; Davidson et al., 1998; Luo et al., 2001), residue lignin/nitrogen ratio (Geng et al., 1993; Melillo et al., 1989), and the role of different microbial types in decomposition process. Portions of root and litter that resist decomposition develop into a stable form of SOM that can last for hundreds and thousands of years before it is broken down by microbes (Luo and Zhou, 2006). However, only about 10% of soil surface CO2 efflux is derived from decomposition of older, more recalcitrant C compounds (Gaudinski et al., 2000). When coupled with potential C input from above- and belowground biomass, C loss from soil surface as CO2 efflux can be used to determine annual C sequestered as shown by researchers using a C budgeting approach (Dugas et al., 1999; Frank and Dugas, 2001; Flanagan et al., 2002; Yazaki et al., 2004; Kucharik et al., 2006).


During a long-term experiment (20–50 yr), most of the SOC sequestered in early years could be released in later years and before the end of the experiment. Therefore, the SOC sequestered during the long-term experiment is not in permanent storage. This loss will accelerate each year with some of the previous net SOC storage being released in subsequent years as a result of tillage disturbance during planting or synthetic N application leading to change in soil environment and subsequent change in soil microbial activities and increase in mineralization process, which would effectively reduce SOC sequestration rate over time (Fig. 4). The interaction between management practices and soil microbial community is complex, but it plays a significant role in influencing the pathways of SOC retention and loss.

Identifying Appropriate Experimental Design

Identifying proper experimental design and treatments are critical considerations in addressing the question of soil C sequestration in any tillage and cropping systems experiments. Chappell et al. (2013) found that measuring an increase in SOC stocks over a C estimation area over time is a prerequisite to demonstrating that SOC has been stored in the land unit. Given the well-known spatial variability and dynamic nature of soil C, the inclusion of a pretreatment baseline particularly in tillage studies is essential to monitor over time the different tillage and cropping systems effects on the rate of SOC sequestration. Many current studies have been established with omission of such consideration particularly in the case of evaluating NT as compared with intensive tillage systems for SOC sequestration. However, it is possible to somewhat overcome this deficiency if time-sequence samples are taken. It is suggested that these be taken on a temporal scale of a minimum of yearly sampling. The underlying assumption in paired comparison studies is that intensive tillage is at a steady state, which may or may not be the case. Significant body of published research used paired comparison between various tillage treatments with one treatment, such as MP, used as baseline or control is in steady state

Fig. 4. Cumulative soil CO2–C effluxes for each site for 2006 and 2007 growing seasons (Guzman and Al-Kaisi, 2010: J. Environ. Quality 39:136-146). www.soils.org/publications/sssaj

may be in error (Franzluebbers, 2010; Johnson et al., 2005; Liebig et al., 2005; Franzluebbers, 2005; Franzluebbers and Follett, 2005; Lal et al., 1998). Moreover, these comparisons are often sampled only once during or at the end of a long-term study to determine the amount and rate of SOC sequestration. If the assumption of steady state is true then any increase in SOC of the comparison treatment (NT) with above baseline treatment (MP) at the end of study would represent the amount of SOC sequestered. Without a pretreatment baseline SOC values (MP), the SOC sequestration magnitude and rate findings cannot be verified (Olson, 2010; Fynn et al., 2009; Sanderman and Baldock, 2010a, 2010b). It has been shown that the SOC sequestration rate based on a NT comparison with SOC stocks of the baseline (MP) treatment in the final study year (West and Post, 2002) might not be correct if baseline (MP) treatment is not at a steady state. It is possible that all treatments including baseline treatment (MP) lost SOC over time (Olson, 2010) and if so the differences between treatments at the end of the study means that one treatment (NT) lost less SOC or retained more SOC in storage and it is losing SOC at a lower rate. Therefore, any loss in pretreatment SOC stocks for baseline treatment (MP) during the long-term experiment must be subtracted from the comparison treatment (NT) SOC stocks gain since the MP baseline is not at a steady state.

Standardized Procedure for Measuring Soil Organic Carbon Stocks

There is a need for a standardized approach to measuring SOC concentration with respect to SOC sequestration, and pre-experimental sampling is suggested. In this regard a paired comparison design should be approached carefully. Paired comparison can work for many agronomic and soil determinations, but may not be ideal for SOC sequestration. Paired comparisons are a valid approach if the objective is to determine differences between treatments in a soil property after a long-term study and the difference can be determined in the last year if replications are sufficient. It is often reasonable to assume each treatment, such as different tillage treatments in a randomized design, had the same level of a property, such as SOC concentration, at the beginning of the study even if that concentration is not known. The pair comparison method has been used successfully in many agronomic studies where the change in soil property by treatment or yield response difference between treatments is important (Olson et al., 2013). However, this is not the case for SOC sequestration since the difference between treatments only highlights gain or loss in SOC stocks over certain time. Losses in SOC stocks can be associated with water erosion, conventional tillage soil disturbance, and disturbance during NT planting, intensification of mono-cropping systems, disturbance during N injection, leaching of soluble SOC, aeration, and SOM mineralization. This can and often does offset the gains in SOC stocks from the aboveground biomass, root system, and crop residue C inputs to the humus or SOM. 353

Previously eroded soils with low SOC concentration were identified as having significant potential to sequester SOC (Lal et al., 1998; Lal, 1999). In a 20-yr study of previously eroded soils on 6% slopes with low SOC concentration, the amount and rates of SOC storage and retention as a result of a conversion to NT or CP tillage systems using the comparison method with MP tillage SOC stocks as a baseline have also been quantified (Olson, 2010). The findings of this study showed no SOC sequestration occurred in the NT, CP, and MP plots since the SOC stocks of the plot area was higher at the start of the experiment than at the end of the study (Table 2). The NT plots did retain or store more SOC stocks after 20-yr than MP. However, there was no increase in sequestered SOC in NT system as compared with pretreatment baseline SOC stocks of the same treatment plots. Any loading of external SOC rich sediment or materials (not directly from the atmosphere through unit plants to the soil humus) such as sediment deposition must be accounted for in the analysis and should be deducted from the amount claimed as SOC sequestration since it does not satisfy the definition or concept of SOC sequestration. Organic amendments represent OC source that was fixed in a different location and transported and applied to the land unit and incorporated into existing SOC (Schlesinger, 1999). Such practice does not involve any transfer or storage of atmospheric CO2. On the contrary it can be source for CO2 emission due to OC decomposition. It is the stored SOC that is transported and applied to the plots from outside the land unit that can be considered retained C. The addition of C amendments to a land unit is not likely a very stable C pool and would most likely increase the amount of CO2 emission from the land unit during decomposition to the atmosphere.

Dynamics and Contribution of Agricultural Management Practices to Soil Organic Carbon Storage over Time

There is a growing body of scientific literature related to the contribution of agronomic best management practices (BMPs) to SOC sequestration (Huggins et al., 2007; Luo et al., 2010, Sanford et al., 2012; Kumar et al., 2012, Van Doren et al., 1976; Pierce and Fortin, 1997; Lyons et al., 1997; Hendrix,

1997; Olson, 2010). These researchers found that SOC stocks declined from pretreatment levels even with long-term NT studies. Sanford et al. (2012), evaluated the influence of six cropping systems on SOC stocks at the Wisconsin Integrated Cropping System Trial over a 20-yr period. Analysis of SOC on either a concentration or mass per volume of soil basis indicated a significant decline (lost 5.5 Mg C ha-1 or 0.28 Mg ha-1 yr-1) across all of the systems. Results from this study demonstrate the importance of: (i) comparing current and initial soil samples when evaluating SOC sequestration and (ii) evaluating changes in SOC stocks throughout the entire soil profile and not limited to the top depth. The losses of SOC stocks at depths below the tillage zone are either a result of loss of SOC inputs from roots, translocation or due to oxidative loss at these depths or both. In Ohio, SOC data were collected from Wooster and Hoytville plot areas before establishments of the tillage treatments (Van Doren et al., 1976). It was reported that before the establishment of the Wooster tillage plots in 1962 the SOC concentration was 14 g kg-1 for the 0 to 15 cm (Ap horizon). The Hoytville plots had a 22 g kg-1 SOC concentration for the 0 to 15 cm (Ap horizon). In 2011, Kumar et al. (2012) reported SOC data for the same sites for PT treatments for the 0- to 40cm layer in 10-cm intervals. One can adjust these values for comparison between 1962 and 2011 by proportionally weighting the 2011 SOC values by combining the 0- to 10-cm top layer with half of the 10- to 20-cm layer. The adjusted 2011 SOC concentrations for the adjusted 0- to 15-cm layer are estimated to be 12.5 g kg-1 in Wooster PT plots and 14.3 g kg-1 in the Hoytville plots (Olson, 2013b). Based on the measured SOC concentrations from Van Doren et al. (1976) in 1962 and 1964, and the 2011 data from Kumar et al. (2012), one can estimate a loss in the Wooster plots of 1.5 g kg-1 (11%) and in the Hoytville plots of 7.7 g kg-1 (35%). These data suggest that the Wooster and Hoytville PT plots were not at a steady state and were losing SOC at a significant rate, 11 and 35%, respectively. This finding does not support the use of PT as the baseline for NT when determining SOC sequestration. The exact amount of net SOC sequestration can only be documented with pretreatment SOC concentration and bulk density measurements which were not reported. There are en-

Table 2. Twenty-year effects of tillage treatments (six replications) on soil organic C (SOC; Mg C ha-1 layer-1) of the Grantsburg soil. Paired comparison with 2009 MP baseline and pretreatment 1988 baseline methods (Olson, 2010). Tillage treatment†

NT

Depth cm 0–15 15–75 0–75 (all)

September 1988 (pretreatment baseline)

June 2009

——Mg C ha-1 layer-1—– 28.5a** 25.2a** 23.6a 20.1a 52.1a 45.3a

Pretreatment 1988 baseline method 20-yr SOC loss (below pretreatment 1988 baseline)

Paired comparison method with MP as baseline NT vs. MP 20-yr SOC retention rate difference (NT above 2009 MP baseline)

—————Mg C ha-1 layer-1————-0.17a +0.40 -0.18a +0.06 -0.35a +0.46

MP

0–15 28.3a 17.3b -0.55b 15–75 23.1a 18.9a -0.21a 0–75 51.4a 36.2b -0.76b ** Mean of six replications with the same letter and in the same year and depth with a different tillage treatment are not significantly different at P = 0.05. † NT, no-till; MP, moldboard plow tillage. 354


vironmental conditions and production input variables that could result in continuous variations in the rate of SOC concentration or stock gains and losses over time, but these were not reported. Furthermore, in 2011 the Wooster PT plots were still showing a 37% SOC loss when compared with a woodlot (WL) and Hoytville PT plots are still showing a 60% SOC loss when compared with WL (Olson, 2013b). If the PT was truly at steady state then all this SOC loss had to occur in the 25 to 30 yr (between 1930s and 1962 or 1964) before the tillage treatments were applied. After 47 and 49 yr of NT treatments, the losses were only 32 and 55%, respectively or a reduction of 5%. This comparison suggests that SOC sequestration is less than reported by Kumar et al. (2012). This example highlights the need to use pre-treatment SOC concentrations or stocks for C sequestration studies. Support for treatment of C sequestration should be a net increase in SOC storage during and at the end of a study above the pretreatment baseline SOC stocks (Olson, 2013b). Huggins et al. (2007), in a study of tillage and crop rotations effects on SOC in southern Minnesota, reported losses of 3.7 and 1.6 Mg SOC ha-1 for low productivity or aggressively tilled soils and high productivity or minimally tilled systems, respectively. These findings were supported by Luo et al. (2010) in a meta-analysis of 69 paired experiments, where they found that conversion from conventionally tilled to NT farming practices affected the SOC stocks location, but not the amount of SOC (